Mark Hamill says that while none of the cast has officially signed on, they are in talks with Lucasfilm's Kathleen Kennedy and writer Michael Arndt. George Lucas has said if they weren't interested,they would be written out, not replaced with another actor. Hamill wants 'the whole gang' to return; Fisher, Ford, Billy Dee Williams, and Anthony Daniels; "... I want to make sure that everybody's on board here, rather than just one." He sees himself returning in an 'Obi-Wan/mentor' role, with a focus on the Skywalker and Solo children. Hamill also hopes the films "go retro", with a return to the lighthearted and humorous tone of the original trilogy, with the "right balance of CGI with practical effects."
Source: Movie Web